Baking Multiple Materials into one Object
I tried to bake with basic materials and it worked fine. you should select same image texture data block in every material node. And also we bake normals and diffuse etc separately rather than combined.

->create a new blank image in uv editor and name it (say bake.png).

-> in node editor for any material: shift+A-> image texture-> browse the created image (which is blank right now).

-> Now copy the image texture node (by cltr+c) go to another material and paste in node editor (by cltr+v) do this to every material you want to bake.

-> make sure you selected right image texture node (bake.png) in every material. Because if you select other image texture node  data will be baked into that image.
-> bake type to diffuse or combined or normals and hit bake, after baked, save image as combined.png  you may need to turn off indirect lighting in bake panel.